1. Field of the Invention
The present invention relates generally to amplifiers, and more particularly to a system and method of employing variable magnetic flux bias in an amplifier.
2. Description of the Related Art
The output power and distortion of an audio frequency amplifier using an output transformer is partially dependent on the specific magnetic properties of the output transformer. Output transformers are commonly used with vacuum tube amplifiers. Such output transformers are used to match the higher impedance of the vacuum tubes to the typically lower impedance of the loudspeakers. Output transformers may also be used for isolation of the amplifier from the load. In these amplifiers, the output transformer may saturate (i.e., saturation of the core) due to the direct current (DC) and low-frequency subsonic currents flowing in the primary of the output transformer. Thus, amplifier designs using an output transformer may suffer from a lack of fidelity and loss of power at the lower-frequency end of the audio spectrum.
The desired signals, those being amplified, are responsible for an alternating magnetic flux component within the output transformer. This alternating magnetic flux follows a magnetic path centered at the quiescent magnetic bias level. The quiescent magnetic bias level is due to the DC and low-frequency subsonic current flowing in the output transformer's primary winding. If the quiescent magnetic bias level is not close to a zero level, the alternating magnetic flux swing within the output transformer's core may cause magnetic saturation. Saturation of the output transformer is undesirable because a greater magnetic flux swing is needed within the output transformer to maintain output power and minimize total harmonic distortion at lower signal frequencies.
One prior art technique addresses the problem of magnetic saturation of the output transformer in an amplifier by placing a physical gap within the output transformer's core. The physical gap significantly lengthens the magnetic path, increasing the magnetic reluctance and shearing the magnetic loop, and thus decreasing the quiescent magnetic flux level. However, this technique reduces the relative magnetic permeability of the transformer's core and adversely affects inductance and the low-frequency response of the output transformer.
A second prior art technique disclosed by Arnold [U.S. Pat. No. 1,448,550] uses a shunt feed capacitor to block DC from passing through the output transformer's primary winding. However, the shunt feed technique suffers from the requirement for a physically large and expensive DC blocking capacitor. The use of smaller shunt feed capacitors adversely effects low-frequency amplifier response to such a degree that shunt feed amplifier systems in the audio market perform only marginally better than amplifier systems employing a physical gap.
A third prior art technique disclosed by Larson [U.S. Pat. No. 5,422,599] uses a current sensing resistor. The current sensing resistor is used to measure the DC and low-frequency current flow in a first primary winding, and to produce in a second primary winding a magnetomotive force equal but opposite to the magnetomotive force in the first primary winding. In this technique, the voltage drop across the current sensing resistor is coupled to a voltage-controlled current source. The voltage-controlled current source couples to the second primary winding and causes opposite DC and low-frequency currents to flow in the second primary. The second primary winding thus may partially cancel the DC and low-frequency magnetomotive force produced by the first primary winding.
However, this technique using a current sensing resistor suffers from several problems, including increased distortion due to the placement of the current sensing resistor in the plate or cathode circuit of the amplifier. In addition, knowledge of the winding ratio between the first primary winding and the second primary winding is required to scale the voltage-controlled current source so as to cancel the DC and low-frequency magnetomotive force produced by the first primary. In practical applications, asymmetries in output transformer windings due to manufacturing tolerances may result in only partial cancellation of the DC and low-frequency magnetomotive force produced by the first primary winding. Finally, this technique does not correct for remnant induction in the output transformer.
Therefore, what is needed are improved techniques that prevent saturation of the output transformer in an amplifier and/or that maintain fidelity and output power at the lower-frequency end of the audio spectrum.